It is important to understand that there are different types of RFID systems, each of which has different characteristics. Passive high-frequency (HF) readers have a well-defined read field, meaning a tag just outside the field would not be read.
The November/December 2010 issue of our print magazine includes an article about an architectural-modeling tool comprising a table with 960 HF readers based on the ISO 15693 air-interface protocol standard, as well as motion-capture technology and RFID-tagged objects, such as buildings, trees, lamp poles and so forth (see Designing in Three Dimensions). The interrogators each cover a small area of the table, so that when a building with an RFID tag is moved to another spot on the table, the tag is read and that object’s location is displayed on a computer screen.
Passive ultrahigh-frequency (UHF) systems lack a well-defined read field. The field created by a circular polarized UHF reader antenna is generally shaped like an American football, with the point of the ball starting a few inches in front of the antenna. A tagged object located 25 feet from the antenna might be read some of the time, but might be missed at other times. There are also null spots, or blind spots, in the read field, caused by multipathing—radio waves bouncing off walls, ceilings and floors when returning from tagged objects, thereby making it difficult to precisely calculate where the signals originated.
If you were to point four reader antennas out from the corners of a room, you would not obtain uniform coverage, nor would the area of overlap be entirely consistent. But whether or not such a system would work for your purposes would depend on what you are trying to achieve. It might be possible to know if an item was located in one area or another, depending on which antenna picks up the tag’s signal.
Another option would be to employ an active ultra-wideband (UWB) solution. UWB systems don’t have multipath issues, and can locate a tagged object to within a few centimeters. However, the tags are expensive.
If you wish to contact me privately to discuss your requirements in more detail, I would be happy to provide advice regarding which solutions might work best for your particular needs.
—Mark Roberti, Founder and Editor, RFID Journal